Boss to read endlessly (4)
I believe the answer is:
stud
'boss' is the definition.
('stud' can be a synonym of 'boss')
'read endlessly' is the wordplay.
'read' becomes 'study' ('study' can be a synonym of 'read').
'endlessly' means to remove the last letter.
'study' with its final letter taken off is 'STUD'.
'to' is the link.
(Other definitions for stud that I've seen before include "Boss; shirt fastener" , "Stallion for breeding - fastener" , "Clothes fastener in horse establishment" , "Animal breeding farm" , "gambling with cards" .)